This invention relates to rotor blades of the type used in industrial gas turbine engines, and more specifically, to the tip region of such a rotor blade.
Gas turbine engines for aircraft have rotor blades that typically are smaller than rotor blades used in, for example, the turbine of an industrial gas turbine that employs steam as a working medium.
The rotor assembly employs such blades with a rotating structure, such as a rotor disk, having an axis of rotation and a plurality of outwardly extending blades. Each blade is disposed about a spanwise axis that extends radially. Generally, the spanwise axis is a radial line referred to as the stacking line which extends outwardly on a radius from the axis of the rotor blade. The rotor blade has a base, commonly called a root, which engages the rotating structure at the inner end of the blade.
The rotor blades each have an airfoil which extends outwardly from the root across the working medium flowpath. The rotor blade typically has a shroud extending between airfoils of adjacent rotor blades at the tip region of the rotor blade. The shroud has cantilevered wings which extend laterally (circumferentially) between adjacent rotor blades. The wings include a portion of a transition zone that extends from the junction with the airfoil and that has an inwardly facing surface which bounds the working medium flowpath. The shroud also has a seal land which extends circumferentially in close proximity to adjacent stator structure to block the working medium gases from leaving the flowpath. In some constructions, a more rigid member extends between the front and rear portions of the wings to carry the seal land and provide a portion of the transition zone.
The shrouds of adjacent rotor blades abut at contact areas on the laterally facing sides of the shroud. The abutting shrouds reduce blade deflections about the spanwise axis and minimize vibration of the rotor blades. Damping of the blades takes place through rubbing of the contact faces of adjacent shrouds. Additional rotational loads are created by the mass of the shroud as compared with rotor blades having no shrouds. These rotational loads increase stresses at the shroud airfoil interface because of the sudden change in cross-section of the material; and, increase stresses at the root-disk interface of the rotor blade and the disk. The stresses in the airfoil and the shroud of the rotor blades require heavier designs than non-shrouded blades of equivalent cyclic fatigue life. In addition, the mass of the shroud may cause creep of the airfoil and creep of portions of the shroud in a radial direction because of rotational forces generated under operative conditions.
Accordingly, scientists and engineers working on the direction of applicants' assignee have sought to develop shrouds for rotor blades that reduce the concentrated stresses in the rotor blades and demonstrate acceptable resistance to creep without causing additional creep in the airfoil by increasing the mass of the rotor blade.